I was wondering in a straight up fight with no missed shots how the BR, DMR, and Carbine compare? Is it like Halo 3 where the combine shoots faster, but time wise needs 1 more shot? Basically if you start shooting at the exact same time and every shot lands which gun wins? I haven't had a chance to test this yet.
With the four rifles in the Halo 4 arsenal, there is actually a very good balance. They will all kill a player in nearly the exact same amount of overall time if every shot hits as fast as the trigger can be pulled and is a head shot.
The DMR, BR, and unzoomed Light Rifle all take 5 trigger-pulls to do this, and all of these fire at very similar rates. A zoomed-in Light Rifle fires a bit slower, but will kill with only 4 headshots. The Carbine has been tweaked a bit since Halo 3, and fires much more quickly than the other weapons, but does significantly less damage. This balances out to a very similar overall time, but with 8 head shots required.
So overall, the Carbine is probably the most difficult to do it with since it requires 8 shots on target, but there are fractions of a second in difference between the overall times for all the rifles.
Major League Gaming created a video actually for these three guns. This is the video: http://tv.majorleaguegaming.com/videos/97756-halo-4-primary-rifle-breakdown-mlg-dr-pepper-ultimate-access-first-look
Battle Rifle - Takes 7 shots at the body to kill the opponent. When the shield is gone it takes 1 head shot to kill the guy. 5(with a full shield) shots to the head will kill an opponent.
DMR - Takes 7 shots at the body to kill the opponent. 5(with a full shield) shots to the head will kill an opponent.
Carbine - Takes 12 shots at the body to kill the opponent. 8(with a full shield) shots to the head will kill an opponent.
Seems like Battle Rifle or DMR are your best bet. Watching a lot of online play it seems like most pro's use the DMR